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CLAYSClays (differentiating the term "clay minerals") are clastic sedimentary rock and soil composed of one ormore clay minerals, with traces of oxides and organic matter. Clays are plastic due to water content andbecome compact, brittle and non-plastic when dry. Clays are generally deposits of phyllosilicatecontaining variable amounts of water trapped in its structure. Depending on the chemical content of thesoil where they are found, the clays can have different colors varying from white, brown, orange or red.

Figure 3. Quaternay deposit of clays (left), color of clays (right).

Clay minerals

Clay minerals commonly form as breakdown products of feldspars and other silicate minerals. They arephyllosilicates with a layered crystal structure similar to that of micas and compositionally they arealuminosilicates. The crystal layers are made up of silica with aluminium and magnesium ions, withoxygen atoms linking the sheets. Two patterns of layering occur, one with two layers, the kandite group,and the other with three layers, the smectite group.

Kaolinite is the commonest member of the kandite group and is generally formed in soil profiles in warm,humid environments where acidic waters intensely leach bedrock lithologies such as granite. Clayminerals of the smectite group include the expandable or swelling clays such as montmorillonite, whichcan absorb water within their structure. Montmorillonite is a product of more moderate temperatureconditions in soils with neutral to alkaline pH. It also forms under alkaline conditions in arid climates.Another three-layer clay mineral is illite, which is related to the mica group and is the most common claymineral in sediments, forming in soils in temperate areas where leaching is limited. Chlorite is a three-layer clay mineral that forms most commonly in soils with moderate leaching under fairly acidic

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groundwater conditions and in soils in arid climates. Montmorillonite, illite and chlorite all form as aweathering product of volcanic rocks, particularly volcanic glass.

Figure 4. Laminar structure of clays

Clay minerals typically form over long periods of time as a result of the gradual chemical weathering ofrocks, usually silicate-bearing, by low concentrations of carbonic acid and other diluted solvents. Thesesolvents, usually acidic, migrate through the weathering rock after leaching through upper weatheredlayers. In addition to the weathering process, some clay minerals are formed through hydrothermalactivity. There are two types of clay deposits: primary and secondary. Primary clays form as residualdeposits in soil and remain at the site of formation. Secondary clays are clays that have been transportedfrom their original location by water erosion and deposited in a new sedimentary deposit. Clay depositsare typically associated with very low energy depositional environments such as large lakes and marinebasins.

Uses of clays

Ball clays are good quality clays used mostly in pottery but are also added to other clays to improve theirplasticity. Ball clays are not as common as other clay varieties. One third of the ball clay used annually isused to make floor and wall tiles. It is also used to make sanitary ware, pottery, and other uses.

Bentonite is formed from the alteration of volcanic ash. Bentonite is used in pet litter to absorb liquids. Itis used as a mud in drilling applications. It is also used in other industrial applications such as the"pelletizing" of iron ore.

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Common clay is used to make construction materials such as bricks, cement, and lightweightaggregates.

Fire clays are all clays (excluding bentonite and ball clays) that are used to make items resistant toextreme heat. These products are called refractory products. Nearly all (81%) of fire clays are used tomake refractory products.

Fuller's earth is composed of the mineral palygorskite (at one time this mineral was called "attapulgite").Fuller's earth is used mostly as an absorbent material (74%), but also for pesticides and pesticide-relatedproducts (6%).

Kaolinite is a clay composed of the mineral kaolin. It is an essential ingredient in the production of highquality paper and some refractory porcelains.

Clay production

Clays are common all over the world. Some regions, as might be expected, produce large quantities ofspecific types of clay. It is estimated that the state of Georgia has kaolin clay reserves of 5 to 10 billiontons. The United States is self-sufficient so it imports only small amounts of clay from Mexico, Brazil,United Kingdom, Canada, and assorted other nations. The United States exports nearly half of itsproduction worldwide.

The nations producing the most significant amounts of the various clays are as follows:

Kaolin: Brazil, United Kingdom, and the United States are the dominant producers of high quality kaolin. Ball clays Major producers of ball clays are Germany, the United States, United Kingdom, the Czech Republic, China, and France. Fire clays Major fire clay producing countries are Germany, and the United States. Bentonite Major producers of bentonite are the United States, Germany, Turkey, and Greece. Fuller's earth: Major producers of fuller's earth are the United States (attapulgite, smectite), Spain (attapulgite, sepiolite), and Senegal (attapulgite).